Method for the production of Grignard compounds

ABSTRACT

The invention relates to a method for producing Grignard compounds, according to which magnesium is reacted in a suitable fluid reaction medium in a protective atmosphere with hologen-substituted organic compounds by means of microwave radiation.

[0001] The present invention relates to a process for the production of Grignard compounds, according to which magnesium is reacted in a suitable liquid reaction medium under a protective gas atmosphere with halosubstituted organic compounds with microwave irradiation.

[0002] Over the past few decades, organometallic compounds have proven to be an indispensable reaction component in the synthesis of organic compounds both in the laboratory and in large industrial scale production despite their sometimes high sensitivity to atmospheric oxygen and moisture.

[0003] Organomagnesium compounds, which also include so-called Grignard compounds, have gained particular significance. These Grignard compounds may be obtained by reacting magnesium with halosubstituted organic compounds under a protective gas atmosphere in a suitable reaction medium and comprise a strongly polarised magnesium-carbon bond, in which the carbon atom bears the negative charge density, see for example, “Grignard Reagents: New Developments”, Herman G. Richey Jr., John Wiley and Sons Ltd, 2000, pages 185 to 275, “Organomagnesium Methods in Organic Synthesis”, Basil J. Wakefield, Academic Press, London, 1995, pages 21 to 71 and “Handbook of Grignard Reagents”, Gary S. Silverman, Philip E. Rakita, Marcel Dekker, Inc., New York, 1996, pages 53 to 77.

[0004] The Grignard compounds therefore readily react as nucleophilic reagents with electrophilic compounds with formation of a new carbon-carbon-bond and are thus suitable for synthesis of larger organic compounds from appropriate fragments.

[0005] In practice, the production of Grignard compounds frequently proves difficult, as the surface of the magnesium is conventionally passivated by a magnesium oxide layer, which markedly impairs or even prevents reaction of the magnesium with the halosubstituted organic compound.

[0006] Processes have therefore been developed to activate the surface of the magnesium. Examples are chemical activation by mineral acids, elemental iodine or and by lithium salts, together with physical activation by heating or by ultrasound treatment.

[0007] However, a disadvantage of these processes for activating the magnesium is that, due to the long reaction times and optionally the elevated reaction temperatures, undesired secondary products are often formed in addition to the desired Grignard compound, such as for example hydrocarbon compounds by the reaction of two halosubstituted organic compounds through Wurtz coupling.

[0008] These secondary reactions lead on the one hand to a reduction in the yield of the desired Grignard compound and thus also to a reduction in the yield of the derived product produced therefrom. On the other hand, these secondary reactions frequently also impair or prevent the use of Grignard compounds in a synthesis, since the subsequently necessary purification steps make the process uneconomic or cannot be performed at all due to small substance quantities, such as for example in the field of combinatorial chemistry.

[0009] The object of the present invention was accordingly to provide a process for the production of Grignard compounds, with which these compounds are obtained within short reaction times and in high yields. Moreover, the obtained Grignard compounds should preferably also be distinguished by good to very good purity.

[0010] According to the invention, this object is achieved by a process for the production of at least one Grignard compound, by reacting magnesium in a suitable liquid reaction medium under a protective gas atmosphere with at least one halosubstituted organic compound, preferably a halosubstituted organic hydrocarbon compound, with microwave irradiation.

[0011] The magnesium is preferably used in the process according to the invention in strip form or in multiparticulate form, particularly preferably in the form of chips or powder. Very particularly preferably, the magnesium is used in the form of conventional, commercially obtainable chips.

[0012] Suitable reaction media which may be used are any reaction media known per se to the person skilled in the art for the production of Grignard compounds, as described for example in “Grignard Reagents: New Developments”, Herman G. Richey Jr., John Wiley and Sons Ltd, 2000, pages 185 to 275, “Organomagnesium Methods in Organic Synthesis”, Basil J. Wakefield, Academic Press, London, 1995, pages 21 to 71 and “Handbook of Grignard Reagents”, Gary S. Silverman, Philip E. Rakita, Marcel Dekker, Inc., New York, 1996, pages 53 to 77. The corresponding literature descriptions are hereby introduced as a reference and are deemed to be part of the disclosure.

[0013] The reaction medium is preferably an ether compound or a mixture containing at least one ether compound. The ether compound is preferably selected from the group comprising aliphatic ethers, cyclic ethers and aliphatic polyethers. Mixtures of two or more representatives of one or more of the above-stated classes of ethers may also be used.

[0014] Examples of preferred aliphatic ethers are diethyl ether, dibutyl ether or mixtures thereof.

[0015] Preferred cyclic ethers are tetrahydrofuran, 1,4-dioxane or mixtures thereof.

[0016] Preferred aliphatic polyethers are based on alkylene glycols, such as for example ethylene glycol, and are particularly preferably selected from the group comprising ethylene glycol dimethyl ether (monoglyme), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme) and mixtures containing at least two of these above-stated polyethers.

[0017] The quantity ratio of halosubstituted organic compound to reaction medium may vary widely. The quantity ratio which is optimum in each case may, where necessary, be determined by the person skilled in the art with the assistance of simple preliminary testing.

[0018] For a good yield of a Grignard compound produced according to the invention, it is advantageous for the reaction medium used to comprise no or at least the smallest possible amounts of oxygen, carbon dioxide and moisture. These gases are therefore preferably removed from the respective reaction medium and/or said medium is dried using conventional methods known to the person skilled in the art prior to the reaction.

[0019] With the process according to the invention, various halosubstituted organic compounds may be reacted simultaneously with the magnesium. Preferably only one halosubstituted organic compound is reacted in each case with the process according to the invention.

[0020] In addition, it goes without saying that, in the process according to the invention, it is possible to use not only organic compounds which comprise one halogen atom as substituent, but also those which are substituted with two or more optionally different halogen atoms.

[0021] The halosubstituted organic compounds preferably used for the reaction according to the invention are aliphatic halogen compounds, cycloaliphatic halogen compounds optionally comprising at least one heteroatom in the ring system or aromatic halogen compounds optionally comprising at least one heteroatom in the ring system.

[0022] Possible saturated or unsaturated aliphatic halogen compounds are those compounds which comprise 1 to 10 carbon atoms, particularly preferably 1 to 5 carbon atoms and very particularly preferably 1 to 3 carbon atoms. Preferred cycloaliphatic halogen compounds, which optionally comprise at least one heteroatom in the ring system, are those with 3 to 8 carbon atoms.

[0023] These halogen compounds may be substituted with fluorine, chlorine, bromine or iodine, preferably with chlorine, bromine or iodine, particularly preferably with bromine or iodine.

[0024] Suitable aromatic halogen compounds are preferably compounds of the general formulae I or II

[0025] with n=1, 2 or 3

[0026] in which X in each case denotes F, Cl, Br or I, preferably Cl, Br or I, particularly preferably Br or I,

[0027] and the substituent R is selected from the group consisting of F, Cl, Br, I, a C₁₋₅ alkyl residue optionally at least mono-substituted with fluorine, a C₁₋₅ alkoxy residue, an NO₂ residue, an N(R¹)₂ residue, a CON(R¹)₂ residue, an SR¹ residue, an SOR¹ residue, an SO₂R¹ residue, an SO₂N(R¹)₂ residue, a residue of the general formulae III to VI

[0028] and an aryl or heteroaryl residue, which may optionally be at least mono-substituted with R′,

[0029] wherein

[0030] A and B, identical or different, denote O, S or NH, and preferably both denote O,

[0031] the residue R¹ in each case denotes a C₁₋₆ alkyl residue, preferably a C₁₋₃ alkyl residue, particularly preferably a methyl or ethyl residue

[0032] or—in the case of the general formula III—the two residues R¹ as ring members mean the group CH₂—CH₂ or CH₂—CH₂—CH₂,

[0033] the residues R², R³, R⁴, R⁵, R⁶ and R⁷, in each case mutually independently, denote H, a C₁₋₆ alkyl residue, preferably a C₁₋₃ alkyl residue, particularly preferably a methyl or ethyl residue, or an optionally substituted aryl residue and/or an aryl residue optionally comprising at least one heteroatom, preferably an optionally substituted phenyl residue, particularly preferably an unsubstituted phenyl residue, and

[0034] R′ is selected from the group consisting of F, Cl, Br, I, a C₁₋₅ alkyl residue optionally at least mono-substituted with fluorine, a C₁₋₅ alkoxy residue, an NO₂ residue, an N(R¹)₂ residue, a CON(R¹)₂ residue, an SR¹ residue, an SOR¹ residue, an SO₂R¹ residue, an SO₂N(R¹)₂ residue and a residue of the above-stated general formulae III to VI.

[0035] If one of the above-stated cycloaliphatic or aromatic halogen compounds comprises at least one heteroatom in the ring system, this may preferably be selected from the group consisting of oxygen, sulfur and nitrogen.

[0036] If a compound of the general formula II is used as aromatic halogen compound, o-bromoanisole, m-bromoanisole, p-bromoanisole, o-bromobenzotrifluoride, m-bromobenzotrifluoride or p-bromobenzotrifluoride, particularly preferably m-bromoanisole, is suitable for the reaction according to the invention.

[0037] The molar ratio of magnesium to halosubstituted organic compound may vary. The magnesium and the halosubstituted organic compound are preferably used in equimolar quantities, because the yield of desired Grignard compound is conventionally achieved thereby.

[0038] The process according to the invention may proceed under the protective gas conditions known to the person skilled in the art and conventional for the production of Grignard compounds. The protective gas atmosphere is preferably an atmosphere of argon and/or nitrogen, as described for example in “Grignard Reagents New Developments”, Herman G. Richey Jr., John Wiley and Sons Ltd, 2000, pages 185 to 275. The corresponding literature description is hereby introduced as a reference and is deemed to be part of the disclosure.

[0039] According to the invention, the power with which the microwaves are irradiated, and the frequency of the irradiated microwaves may likewise vary over a wide range.

[0040] The microwave irradiation preferably proceeds at a power of 100 to 1200 watt, particularly preferably 100 to 250 watt.

[0041] The frequency of the irradiated microwaves is preferably in the range of 850 to 22250 MHz, particularly preferably in the range of 915±25 MHz, 2450±13 MHz, 5800±75 MHz or 22125±125 MHz.

[0042] The duration of the reaction for performing the process according to the invention may vary as a function of a plurality of parameters, for example the type of halosubstituted organic compound, the type of reaction medium and/or the reaction temperature. The duration of the reaction which is in each case optimum may be determined by the person skilled in the art by simple preliminary testing.

[0043] The duration of the reaction preferably amounts, in the process according to the invention, to no more than 60 minutes, particularly preferably no more than 45 minutes and very particularly preferably no more than 30 minutes.

[0044] In a preferred embodiment of the process according to the invention, the reaction proceeds at a temperature of up to at most the boiling temperature of the liquid reaction medium. The reaction is preferably performed with refluxing of the liquid reaction medium.

[0045] The time at which irradiation of the microwaves for the reaction begins and the duration of microwave irradiation may likewise vary.

[0046] Thus, it is possible, for example, to begin microwave irradiation only once all the reaction components have been added to the reaction medium. Alternatively, the magnesium may for example be initially present in the reaction medium, part of the halosubstituted organic compound may be added, then microwave irradiation may be started and then the remainder of the halosubstituted organic compound may further be added.

[0047] Microwave irradiation may for example proceed only up to the start of the reaction of the magnesium with the halosubstituted organic compound, so-called activation of the reaction, or during the entire reaction.

[0048] With the process according to the invention, Grignard compounds may be produced in a very good yield and within short reaction times. The Grignard compounds obtained using the process according to the invention are further distinguished by high purity, such that they are very well suited to the production of chemical compounds which conventionally should not be subjected to any further purification step after production, such as for example substance libraries produced using combinatorial chemistry.

[0049] The invention is explained below with reference to Examples. These explanations are given merely by way of example and do not restrict the general concept of the invention.

EXAMPLES

[0050] The chemicals and reaction media used were purchased from conventional suppliers (Acros, Aldrich, Fluka, Lancaster, Merck).

[0051] The gas chromatography analyses were performed with an HP 6890 gas chromatograph with a PTV (Programmed Temperature Vaporization (Inlet)) injector and a 5973 mass selective detector (Hewlett-Packard, now Agilent Technologies Deutschland GmbH, Hewlett-Packard-Straβe 8, 76337 Waldbronn) coupled thereto or with a GC 17 A gas chromatograph (Shimadzu Deutschland GmbH, Albert-Hahn-Straβe 6 bis 10, 47269 Duisburg).

Example 1a Production of Grignard Compound From m-bromoanisole

[0052] In a microwave device of the type MLS ETHOS 600 (MLS-GmbH, Auenweg 37, 88299 Leutkirch, Germany), an apparatus was constructed which consisted of a 250 ml two-necked flask with a temperature sensor and a 40 cm long dip tube, onto which were fitted a two-necked attachment with a dropping funnel with pressure equalisation and a jacketed reflux condenser together with an inlet for protective gas.

[0053] 50 mmol of magnesium chips in 20 ml of dried, nitrogen-bubbled tetrahydrofuran were then introduced into the two-necked flask and the apparatus was provided with a nitrogen atmosphere. Under 250 watt microwave irradiation, 1 g of m-bromoanisole (corresponding 10 wt. % of the total quantity of m-bromoanisole) was added dropwise. The mixture obtained in this way was then further irradiated with microwaves at a power of 100 watt. After activation of the reaction—the reaction medium became turbid—the remaining quantity of the m-bromoanisole was added dropwise and irradiation with microwaves was continued at a power of 100 watt.

[0054] At intervals of in each case five minutes, beginning once addition of all the m-bromoanisole was complete, samples were taken with a syringe, which were then introduced into a saturated ammonium chloride solution for hydrolysis of the Grignard compound produced. The aqueous solution thus obtained was then extracted with the assistance of diethyl ether and analysed by gas chromatography, optionally in combination with mass spectrometry. This analysis provides information about the content of Grignard compound produced, unreacted halogen compound and undesired secondary products.

[0055] The determined quantities of the anisole produced by hydrolysis of the Grignard compound, unreacted m-bromoanisole and the quantity of undesired secondary products are reproduced in Table 1a below: TABLE 1a Duration of % Secondary reaction in minutes % Anisole % m-Bromoanisole products 5 66 23 11 10 78 12 10 15 89 7 4

Example 1b Production of Grignard Compound From m-bromoanisole

[0056] The structure, procedure and analysis were as set out under Example 1a. At variance with Example 1a, microwave irradiation proceeded only once addition of the entire quantity of m-bromoanisole was complete. The duration of microwave irradiation was 30 minutes at a power of 100 watt.

[0057] The determined quantities of anisole produced by hydrolysis of the Grignard compound, unreacted m-bromoanisole and the quantity of undesired secondary products are reproduced in Table 1b below: TABLE 1b Duration of % Secondary reaction in minutes % Anisole % m-Bromoanisole products 5 80 15 5 10 93 6 1 15 96 4 — 20 96 4 — 25 99 1 —

Example 1c Production of Grignard Compound From m-bromoanisole

[0058] The structure, procedure and analysis were as set out under Example 1a. At variance with Example 1a, 100 watt microwave irradiation proceeded only until activation of the reaction. Stirring was then continued for 30 minutes at room temperature.

[0059] The determined quantities of anisole produced by hydrolysis of the Grignard compound, unreacted m-bromoanisole and the quantity of undesired secondary products are reproduced in Table 1c below: TABLE 1c Duration of % Secondary reaction in minutes % Anisole % m-Bromoanisole products 28 88 12 —

Example 2 Production of Grignard Compound From m-bromoanisole

[0060] The structure, procedure and analysis were as set out under Example 1a. At variance with Example 1a, 20 ml of dried dioxane were used as reaction medium. The determined quantities of the anisole produced by hydrolysis of the Grignard compound, unreacted m-bromoanisole and the quantity of undesired secondary products are reproduced in Table 2 below:

Comparative Example 1 Production of Grignard Compound From m-bromoanisole Without Microwave Irradiation

[0061] 50 mmol of magnesium chips in dried dioxane were introduced under a nitrogen atmosphere into a 250 ml three-necked flask with jacketed coil condenser, dropping funnel with pressure equalisation and protective gas inlet and outlet, 9.56 g (50 mmol) of m-bromoanisole were added dropwise and refluxing was performed for three hours once addition of the m-bromoanisole was complete.

[0062] The reaction mixture was analysed in accordance with Example 1a.

[0063] The determined quantities of the anisole produced by hydrolysis of the Grignard compound, unreacted m-bromoanisole and the quantity of undesired secondary products are reproduced in Table 2 below: TABLE 2 % Secondary Duration of reaction % Anisole % m-Bromoanisole products Example 2  25 minutes 96 2 2 Comparative Example 1 180 minutes No reaction

[0064] While the Grignard compound was produced from m-bromoanisole using the process according to the invention in a very short reaction time with a very good yield and very good purity, production using the conventional process for the production of Grignard compounds does not result in any reaction between the reaction components.

Example 3 Production of Grignard Compound From m-bromobenzotrifluoride

[0065] The procedure was as described for Example 1b. At variance therewith, m-bromobenzotrifluoride was used as halosubstituted organic compound. The duration of microwave irradiation was 55 minutes at a power of 100 to 150 watt.

[0066] The determined quantity of the benzotrifluoride produced by hydrolysis of the Grignard compound, unreacted m-bromoanisole and the quantity of undesired secondary products are reproduced in Table 3 below.

Comparative Example 2

[0067] The procedure was as described for Comparative Example 1. At variance therewith, m-bromobenzotrifluoride was used as halosubstituted organic compound.

[0068] The determined quantity of the benzotrifluoride produced by hydrolysis of the Grignard compound, unreacted m-bromoanisole and the quantity of undesired secondary products are reproduced in Table 3 below. TABLE 3 Duration of % reaction % % m- Secondary in minutes Benzotrifluoride Bromobenzotrifluoride products Example 3  5 74 26 —  10 69 31 —  15 71 29 —  30 50 42  8  40 34 52 14  55 34 52 14 Comparative Example 2 180 minutes 31 33 36

[0069] In addition to an improvement in the yield of desired Grignard compound, production using the process according to the invention results in a smaller quantity of undesired secondary products.

Example 4 Production of the Grignard Compound From p-chlorobenzylchloride

[0070] The procedure was as described for Example 1b. At variance therewith, p-chlorobenzylchloride was used as halosubstituted organic compound and dried diethyl ether was used as reaction medium. The duration of microwave irradiation was 15 minutes at a power of 100 to 150 watt.

[0071] The determined quantity of the benzylchloride produced by hydrolysis of the Grignard compound, unreacted p-chlorobenzylchloride and the quantity of undesired secondary products are reproduced in Table 4 below.

Comparative Example 3

[0072] The procedure was as described for Comparative Example 1. At variance therewith, p-chlorobenzylchloride was used as halosubstituted organic compound and dried diethyl ether was used as reaction medium.

[0073] The determined quantity of the benzylchloride produced by hydrolysis of the Grignard compound, unreacted p-chlorobenzylchloride and the quantity of undesired secondary products are reproduced in Table 4 below. TABLE 4 Duration of reaction % Secondary in minutes % Benzylchloride p-Chlorobenzylchloride products Example 4  5 59 20 21  10 62 9 29  15 72 3 25 Comparative Example 3 180 minutes 45 31 24

[0074] With an unchanged content of secondary products, an excellent yield of the desired Grignard compound may be obtained within a substantially shorter reaction time. 

1. A process for the production of at least one Grignard compound, characterised in that magnesium is reacted in a suitable liquid reaction medium under a protective gas atmosphere with at least one halosubstituted organic compound with microwave irradiation:
 2. A process according to claim 1, characterised in that the magnesium is present in the form of magnesium strip or in multiparticulate form, preferably in the form of chips or powder, particularly preferably in the form of chips.
 3. A process according to claim 1, characterised in that at least one ether compound or a mixture containing at least one ether compound is present as reaction medium.
 4. A process according to claim 3, characterised in that the ether compound is selected from the group consisting of aliphatic ethers, cyclic ethers and aliphatic polyethers.
 5. A process according to claim 4, characterised in that at least one dialkyl ether, preferably diethyl ether, dibutyl ether or a mixture thereof, is present as aliphatic ether.
 6. A process according to claim 4, characterised in that tetrahydrofuran, 1,4-dioxane or a mixture thereof is present as cyclic ether.
 7. A process according to claim 4, characterised in that the aliphatic polyether is based on an alkylene glycol, preferably on ethylene glycol.
 8. A process according to claim 7, characterised in that the polyether is selected from the group consisting of ethylene glycol dimethyl ether (monoglyme), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme) and mixtures containing at least two of these polyethers.
 9. A process according to claim 1, characterised in that an aliphatic halogen compound, a cycloaliphatic halogen compound optionally comprising at least one heteroatom in the ring system or an aromatic halogen compound optionally comprising at least one heteroatom in the ring system is used as a halosubstituted organic compound.
 10. A process according to claim 9, characterised in that the aliphatic halogen compound comprises 1 to 10 carbon atoms, preferably 1 to 5 carbon atoms, particularly preferably 1 to 3 carbon atoms.
 11. A process according to claim 9, characterised in that the cycloaliphatic halogen compound comprises 3 to 8 carbon atoms.
 12. A process according to claim 1, characterised in that the halogen compound is substituted with fluorine, chlorine, bromine or iodine, preferably with chlorine, bromine or iodine, particularly preferably with bromine or iodine.
 13. A process according to claim 9, characterised in that the aromatic halogen compound used is a compound of the general formulae I or II,

with n=1, 2 or 3

in which X in each case denotes F, Cl, Br or I, and the substituent R is selected from the group consisting of F, Cl, Br, I, a C₁₋₅ alkyl residue optionally at least mono-substituted with fluorine, a C₁₋₅ alkoxy residue, an NO₂ residue, an N(R¹)₂ residue, a CON(R¹)₂ residue, an SR¹ residue, an SOR¹ residue, an SO₂R¹ residue, an SO₂N(R¹)₂ residue, a residue of the general formulae III to VI

and an aryl or heteroaryl residue, which may optionally be at least mono-substituted with R′, wherein A and B, identical or different, denote 0, S or NH, and preferably both denote 0, the residue R¹ in each case denotes a C₁₋₆ alkyl residue, preferably a C₁₋₃ alkyl residue, particularly preferably a methyl or ethyl residue or—in the case of the general formula III—the two residues R¹ as ring members mean the group CH₂—CH₂ or CH₂—CH₂—CH₂, the residues R², R³, R⁴, R⁵, R⁶ and R⁷, in each case mutually independently, denote H, a C₁₋₆ alkyl residue or an optionally substituted aryl residue and/or an aryl residue optionally comprising a heteroatom, and the residue R′ is selected from the group consisting of F, Cl, Br, I, a C₁₋₅ alkyl residue optionally at least mono-substituted with fluorine, a C₁₋₅ alkoxy residue, an NO₂ residue, an N(R¹)₂ residue, a CON(R¹)₂ residue, an SR¹ residue, an SOR¹ residue, an SO₂R¹ residue, an SO₂N(R¹)₂ residue and a residue of the above-stated general formulae III to VI.
 14. A process according to claim 13, characterised in that X denotes Cl, Br or I, preferably Br or I.
 15. A process according to claim 13, characterised in that the residue R¹ denotes a C₁₋₃ alkyl residue, preferably a methyl or ethyl residue.
 16. A process according to claim 13, characterised in that the residues R², R³, R⁴, R⁵, R⁶ and R⁷, in each case mutually independently, denote a C₁₋₃ alkyl residue preferably a methyl or ethyl residue, or an optionally substituted phenyl residue, preferably an unsubstituted phenyl residue.
 17. A process according to claim 13, characterised in that A and B in each case denote
 0. 18. A process according to claim 13, characterised in that the compound of the general formula II is selected from the group consisting of o-bromoanisole, m-bromoanisole, p-bromoanisole, o-bromobenzotrifluoride, m-bromobenzotrifluoride and p-bromobenzotrifluoride, preferably m-bromoanisole.
 19. A process according to claim 13, characterised in that p-chlorobenzylchloride is used as compound of the general formula I.
 20. A process according to claim 1, characterised in that the magnesium and the halosubstituted organic compound(s) are used in equimolar quantities.
 21. A process according to claim 1, characterised in that the protective gas atmosphere is an atmosphere of argon and/or nitrogen.
 22. A process according to claim 1, characterised in that microwave irradiation proceeds at a power of 100 to 1200 watt, particularly preferably 100 to 250 watt.
 23. A process according to claim 1, characterised in that irradiation is performed with microwaves of a frequency in the range of 850 to 2250 MHz, preferably in the range 915±25 MHz, 2450±13 MHz, 5800±75 MHz or 22125±125 MHz.
 24. A process according to claim 1, characterised in that duration of the reaction amounts to no more than 60 minutes, preferably no more than 45 minutes and particularly preferably no more than 30 minutes.
 25. A process according to claim 1, characterised in that the reaction proceeds at a temperature of up to at most the boiling temperature of the liquid reaction medium.
 26. A process according to claim 25, characterised in that the reaction proceeds with refluxing of the liquid reaction 